1. Field of the Invention
This invention relates generally to a system and method for selectively providing a freeze purge of a fuel cell stack and, more particularly, to a system and method for selectively providing a freeze purge of a fuel cell stack at system shut-down that considers cell membrane humidification, ambient temperature and other factors.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As is well understood in the art, fuel cell membranes operate with a controlled hydration level so that the ionic resistance across the membrane is low enough to effectively conduct protons. The relative humidity (RH) of the cathode outlet gas from the fuel cell stack is typically controlled to control the hydration level of the membranes by controlling several stack operating parameters, such as stack pressure, temperature, cathode stoichiometry and the relative humidity of the cathode air into the stack. It is known in the art to recover water from the cathode exhaust stream and return it to the stack via the cathode inlet airflow. Many devices could be used to perform this function, such as a water vapor transfer (WVT) unit. By holding a particular set-point for cathode outlet relative humidity, for example 80%, the proper stack membrane hydration level can be maintained.
It is known in the art to provide high frequency resistance (HFR) measurements of the membranes in a fuel cell stack to provide an accurate measurement of the water or membrane hydration in the fuel cell stack. HFR measurement systems provide a high frequency component on the electrical load of the stack, which operates to create a high frequency ripple on the current output of the stack. The resistance of the high frequency component is measured, which is a function of the amount of water in the stack.
At fuel cell system shut-down, it is desirable that the membranes have a certain hydration level so they are not too wet or too dry. This is typically accomplished by purging either the cathode side of the stack or both the cathode and anode side of the stack with dry air for a certain period of time. In one known technique, the purge of the anode side occurs by air being forced through the membranes from the cathode side. Too much water in the stack may cause problems for low temperature environments where freezing of the water could produce ice that blocks flow channels and affects the restart of the system. However, too long of a purge could cause the membranes to become too dry where the membranes will have too low of a protonic conductivity at the next system restart that affects restart performance as well as reduces the durability of the stack. The actual target amount of grams of water in the stack will vary depending on the system and certain system parameters.
For a fuel cell stack having three hundred fuel cells, and an active area near 400 cm2 per cell, the stack may have about two hundred grams of water when the system is shut down. It is desirable that a stack of this size have about twenty-three grams of water after system shut-down so that the membranes are properly hydrated. Twenty-three grams of water is a stack λ of three, where λ represents the membrane hydration defined as the number of water molecules for each sulfonic acid molecule in the membrane for each fuel cell. By knowing how much water is actually in the fuel cell stack at system shut-down, a desirable air purge flow rate and air purge duration can be provided so that the target value of λ=3 can be achieved. Models can be employed to estimate the amount of water in the stack based on stack operating parameters during operation of the fuel cell system.
If a fuel cell stack has too much water in it from the last system shut-down, the water generated during a long start-up may block gas flow channels. Typically, the colder the stack is at start-up the longer it takes to adequately heat up the stack and the more likely that the water generated during start up will block the gas flow channels. Therefore, at very cold start-up temperatures, such as below −15° C., it takes longer for the fuel cell stack to heat up to 0° C. Consequently, the shut-down process becomes very critical for a successful restart of the fuel cell stack, particularly when the fuel cell stack temperature is −15° C. or colder.
The stack purge process when a fuel cell system is being shut-down and a system freeze start-up process require significant hydrogen fuel, which if not needed is wasteful. Certain situations would not require a freeze purge shut-down of a fuel cell stack, such as a short run time or operating in a warm environment. Further, performing the freeze purge to reduce the amount of water in the fuel cell membranes may be desirable for a freeze start-up process, but drying out the membranes to this degree has an adverse effect for membrane durability because increasing and reducing the RH of the membrane, known as RH cycling, causes pin holes to be formed in the membrane, which effect membrane performance. Therefore, from a membrane durability standpoint, it is desirable to limit the number of freeze purges. Thus, if it is known that a freeze condition will not occur when the fuel cell system is off, it is beneficial to not perform a shut-down freeze purge and the subsequent freeze start-up procedure.
One way to determine if a freeze purge is not necessary at system shut-down is to monitor the ambient temperature and if that temperature is not below a certain predetermined temperature, then the freeze purge will not be performed. However, other things, such as time since last key off of the vehicle, are also factors in determining whether the freeze purge will be necessary.